The invention relates, in general, to data communications and data communications systems and devices and, more specifically, to an apparatus and method for adaptively providing optimized forward error correction in a data communications system, where the optimization is based on the predicted occurrence of noise bursts.
The transmission of information in digital form continues to grow at an exponential rate. Analog source information including text, video, audio, and multimedia, is digitized for transmission over a wireless or wire-based communications network. Generally, with the exception of baseband communications schemes, the digital information modulates a radio frequency carrier signal for efficient transmission over the communications medium. The information can be carried over the communications link continuously or in packets, and further can be time or frequency multiplexed to provide more efficient use of the channel by multiple users.
One such specific communications system employing digitally encoded video, voice and other forms of data, is the CableComm(trademark) System currently deployed by Motorola, Inc., of Schaumburg, Ill. In the CableComm(trademark) System, a hybrid optical fiber and coaxial cable provides substantial bandwidth to receiving stations, such as individual subscriber access units located at, for example, households having cable television capability. The coaxial and fiber cables are further connected to fiber optical cables terminating at a central location (referred to as the xe2x80x9chead endxe2x80x9d) where controlling, receiving and transmitting equipment resides. The head end equipment may be connected to any variety of network or other information sources, such as the Internet, online services, telephone networks, and video/movie subscriber services. With the CableComm(trademark) System, digital data can be transmitted both in the downstream direction, from the head end to an individual user, or in the upstream direction, from the user to the head end.
In one embodiment of the CableComm(trademark) System, downstream data is transmitted using 64 quadrature amplitude modulation (QAM) at a rate of 30 Mbps (megabits per second), over 6 MHz bandwidth channels in the frequency spectrum of 88-860 MHz. Anticipating asymmetrical requirements with a significantly greater quantity of data tending to be transmitted in the downstream direction than in the upstream direction, less capacity is provided for upstream data transmission. The upstream channel utilizes PI/4 differential quadrature phase shift keying (PI/4-DQPSK) modulation in the frequency band from 5-42 MHz with a symbol rate of 384 ksymbols/sec with 2 bits/symbol. In addition, the communication system is designed to have a multipoint configuration, i.e., many end users transmitting upstream to the head end, with one or more head end stations transmitting downstream to the end users. The communications system is designed for asynchronous transmission, with users independently transmitting and receiving packets of encoded digital data, such as video or text files. Transmission of this data type are generally bursty, with users receiving or transmitting data at indeterminate intervals over selected channels in accordance with polling, contention, or other protocols established at the head end, rather than transmitting more or less continuously with synchronous streams of information over a dedicated or circuit switched connection.
For asynchronous data transmission, it is desirable to organize the data into recognizable frames or packets for reliable detection by the receiver. In the CableComm(trademark) System, the preamble of the data packet contains timing and synchronization information to ensure accurate data reception and decoding. The timing information is followed by the source information or application information, which may be encoded for both security (encryption) and for error detection and correction. Forward error correction checksum information (in the form of bits appended to the source information or the application information) allows both error detection and error correction.
Impairments in the transmission channel and failures within the communication devices inevitably produce errors in one or more bits, (which are especially troublesome when they occur in the information portion of the transmitted word), and it is therefore desirable to detect and if possible correct such errors during the decoding process at the receiving end. The basic premise of error detection and correction (referred to as forward error correction) is to transmit additional bits, referred to as check bits (or check bytes, checksum bits or forward error correcting bits), in addition to the information data. Forward error correction requires the transmission of more bits than are necessary to transmit only the information, where these bits are appended to the transmitted word so that error detection and correction processes can be executed at the receiving end.
One difficulty arising from the inclusion of error correction information is the attendant increase in overall word or packet size, adding overhead for data transmission and correspondingly decreasing data through-put. Also, the inclusion of error correction information typically increases the system response time or latency, due to the extra time consumed at the receiving end to decode the checksum word, and, if necessary, correct errors in the data word. To constrain the extra error correction overhead, at least to a degree, the number of data errors that can be corrected, referred to as the forward error correcting power, is selected to meet the required performance demands of the communications system. However, the amount of overhead incurred by the system is directly proportional to the selected forward error correcting power. There may be some situations, such as low noise conditions, in which the forward error correcting power is xe2x80x9cexcessivexe2x80x9d, and therefore a higher data through-put can be achieved by reducing the forward error correction power. But, if insufficient correcting power is applied, the overall system through-put performance suffers due to errors in the transmitted data that the correction scheme cannot correct, thereby necessitating the retransmission of the uncorrectable words. Ideally, the correcting power applied should be matched to the impairment level of the communications channel. Various prior art methods are known for providing error correction capability, however, typically these methods utilize a fixed error correction capability, without regard to the specific noise conditions of the channel and the possible opportunities to increase data through-put and decrease response latency when lower than expected noise conditions are present.
Since the impairment or noise levels on a channel may vary with time, an adaptable and flexible error correction capability, which provides sufficient error correction for accurate data reception, while simultaneously minimizing overhead for increased data through-put, is required. U.S. Pat. No. 5,699,365, assigned to the assignee of the present invention, provides a limited degree of adaptable and flexible error correction capability. The apparatus and method disclosed in the patent monitors a parameter of the communications channel and then compares the actual parametric value with a threshold level. If the monitored parameter is not within a threshold value, an additional degree of forward error correction is added to the transmitted bit stream. Disadvantageously, this prior art mechanism lacks the ability to quantitatively determine the optimal forward error correction correcting power to be applied at a given time over a given communications channel. The prior art is focused on revising forward error correction parameters as a result of an error term, specifically bit error, packet error, frame error, etc. Further, such an approach relies on reaching the best solution by slowly adapting to the new noise conditions, it does not make an instantaneous assessment of the best forward error correction configuration. The present invention addresses this problem by characterizing the noise and analyzing the noise characteristics to select the optimized parameters for forward error correction. The present invention does not rely on a traditional feedback control loop that must adapt over time.